If you buy what the tech literati is selling these days, glucose-sensing armbands, heart-monitoring patches, and other wearable electronics will be the next wave of consumer electronic devices. But inventing these devices is only half the battle. Researchers must also come up with flexible, stretchable batteries to power them. Battery researchers have taken a few stabs at it. But most such batteries to date don’t produce much juice.
Now, researchers have engineered a next-generation battery technology, known as lithium-air batteries, into flexible and bendable cablelike cells. The new devices still have a ways to go before they’re ready for market. But someday flexible lithium-air batteries could power everything from clothing packed with light-emitting diodes (LEDs) to roll-up tablets and prosthetic hands.
“There is an urgent need for flexible power sources for next-generation wearables and smart fabrics,” says Venkat Viswanathan, a mechanical engineer and battery technology specialist at Carnegie Mellon University in Pittsburgh, Pennsylvania, who was not involved in the new study. “These [new] design concepts could usher us into the era of truly flexible devices.”
Next-generation lithium-air batteries have long been the siren song beckoning batterymakers. In theory, they can store 10 times as much energy as today’s commercial lithium-ion cells. But among their many problems, when it comes to wearable electronics, is that they’re not flexible. Like other rechargeables, they consist of a negatively charged electrode, called an anode, and a positively charged cathode, separated by a liquid electrolyte that allows lithium ions inside to zip back and forth between the electrodes. Conventional lithium-air cathodes, however, are typically made of rigid materials, such as ceramics encased in delicate fiberglass. And if the battery is flexed, the electrolyte—a liquid—often leaks out. This problem is compounded when oxygen reacts with lithium at the cathode to produce lithium peroxide, a solid that builds up and pushes out the electrolyte, killing the battery cell.
To solve these problems, materials scientist Xinbo Zhang and a team of engineers at the Changchun Institute of Applied Chemistry in China, revamped the way they made their cells. Rather than placing their electrodes side by side, as in a car battery, they arranged them in concentric layers, like a coaxial cable. At the core of the cable in this case is a flexible wire of bare lithium metal. But in place of the liquid electrolyte, Zhang and colleagues fashioned its electrolyte from a pliable polymer gel. They then substituted the normally rigid cathode material with a fabriclike carbon mesh, which itself was encased with the gel and a spongy nickel foam. Lastly, the researchers added a final layer of rubber that contracts when heated. This rubber was punched through with an array of holes to let air inside, which would then diffuse through the nickel foam to spread throughout the carbon fabric cathode. When this assembly was complete, the researchers gently heated the outer rubber layer, causing it to shrink wrap the rest of the components and packing the layers in close electrical contact.
As the scientists report this month in the journal Small, their novel lithium-air battery proved robust and flexible. For starters, it survived bending more than 1000 times even while powering a set of LED lights. It could be recharged 90 times. And, as a bonus, it was waterproof, working up to 5 minutes while submerged. The polymer electrolyte, Zhang and his colleagues explain, is highly hydrophobic, meaning that it blocks the passage of any water that may get in through the holes in the rubber and the porous cathode materials. That’s valuable, Viswanathan and others say, because the lithium metal in conventional lithium-air batteries readily reacts with water and can quickly corrode.
But more work is still needed, says battery chemistry expert Tao Liu from the University of Cambridge in the United Kingdom, who was not involved in the study. Consumers will likely want to charge their rechargeable devices more than 90 times. As well, Liu wonders whether lithium peroxide will hamper the cathode’s conductivity over extended use, as it does with traditional lithium-air batteries.
Viswanathan adds that the flexible materials themselves could also change over time. “When you have materials that are soft [like flexible electrodes], then over time, you could have issues with electrodes that deform,” he says, causing the battery to short-circuit. “But if the electrodes are designed really well,” he adds, “the battery could remain stable for a long, long time.”
Zhang and his colleagues are currently working on improving their electrodes, battery cell configuration, and structural design, which may help address these challenges. There’s still some time before air will help power our coming flexible devices, but now that future appears a bit closer.